Repression of Heme Oxygenase-1 Expression as a Defense Strategy in Humans

Abstract

Heme oxygenase-1 (HO-1) is an essential enzyme in heme catabolism and is characterized by its inducibility in response to various environmental factors, including its substrate heme. The induction of HO-1 has been established as the defense mechanism against oxidative stress. However, striking interspecies or inter-tissue differences are noted in the regulation of HO-1 expression under hypoxia or heat shock, each of which represses HO-1 expression in many types of human cells but rather induces it in rodent cells. The downregulation of HO-1 expression may reduce energy expenditure and local production of carbon monoxide, iron, and bilirubin and transiently increase intracellular heme pool. Here, we discuss the repression of HO-1 expression as a potential defense strategy in humans by highlighting a regulatory role of HO-1 in its own expression.

Heme oxygenase-1 (HO-1) is an essential enzyme in heme catabolism, generating carbon monoxide (CO), iron, and biliverdin/bilirubin. HO-1 expression is induced by various environmental factors, including its substrate heme. The induction of HO-1 has been established as a defense mechanism against oxidative stress. However, HO-1 expression is repressed in human cells origin under thermal stress (1) or hypoxia (2) or by the treatment with interferon-γ (3) or an iron chelator, desferrioxamine (2). Importantly, these repressors for human HO-1 expression could conversely induce HO-1 expression in cultured rodent cells. We therefore hypothesize that the repression of HO-1 expression may represent a defense strategy developed in humans (4), although experimental findings in cultured human cells do not necessarily reflect the human condition.

There are three possible physiological consequences in the repression of HO-1 expression. The repression of HO-1 expression reduces energy expenditure consumed for heme catabolism and also prevents the local accumulation of CO, iron, and bilirubin beyond certain threshold levels in the HO-1-expressing cells and their surroundings (4). These consequences may be important especially in the brain under certain conditions, such as infectious diseases. A third possibility is the feedback regulation mediated by intracellular heme (Fig. 1). The repression of HO-1 expression may transiently increase the intracellular heme level, which in turn regulates gene transcription (5). In this context, a transcription repressor Bach1 has been identified as a mammalian heme-responsive transcriptional regulator, and its repressor function is lost upon heme binding (6). Bach1, a member of basic leucine-zipper factors, functions as a heterodimer with one of small Maf proteins and represses transcription through the Maf recognition element (MARE)(6, 7). Furthermore, expression levels of HO-1 are constitutively higher in various tissues of Bach1-deficient mice (8), suggesting that transcription of the mouse HO-1 gene is under the negative regulation by Bach1. It is therefore conceivable that repression of HO-1 expression may facilitate heme to bind to Bach1, thereby inactivating Bach1’s function as a repressor and derepressing transcription of the Bach1’s target genes, including HO-1. Notably, the human HO-1 gene contains a putative MARE immediately downstream from the cadmium-responsive element (9)(Fig. 1).

Molecular Basis for the Inter-Species Differences in Regulation of HO-1 Expression

The inter-species difference in the regulation of HO-1 expression may be a consequence of the differential expression levels of relevant transcriptional regulators, such as Bach1. In this context, transcription of the mouse HO-1 gene is activated through MAREs by heterodimers of Nrf2, a factor related to nuclear factor erythroid 2, and one of small Maf proteins (10). Thus, availability of Nrf2 and Bach1 may determine whether transcription of the HO-1 gene is activated or repressed (Fig. 1). Alternatively, different arrays of cis-acting elements are responsible for the differential regulation of HO-1 expression; for example, the proximal promoter region of the human HO-1 gene contains the putative silencer sequence for the heat shock element (HSE) (1) and the polymorphic (GT)n repeats (11)(Fig. 1). In rat cells, HO-1 mRNA and activity are increased by heat shock (12), whereas HO-1 expression is not induced by heat shock in human cells (1). We have provided evidence that the sequence flanking the HSE may prevent the heat-mediated activation of the human HO-1 gene.

Conclusions and Prospects

The inter-species variations in the regulation of HO-1 expression between human and rodent cells may reflect the defense strategy uniquely developed in humans after the human-rodent split (13). Implications of the repression of HO-1 expression have been discussed in the relevance to the pathogenesis of severe malaria caused by Plasmodium falciparum (4). Cerebral malaria, the most severe complication of falciparum malaria, manifests as coma and is associated with massive intravascular hemolysis and jaundice. The pathogenesis of cerebral malaria is not fully understood but involves local hypoxia in cerebral microvasculature and high fever. A collaborative study is in progress to analyze the association between the (GT)n polymorphism of the HO-1 gene promoter and the susceptibility to cerebral malaria in Myanmar. We are also working on the mechanism(s) by which HO-1 expression is repressed in human cells under hypoxia or by the treatment with interferon-γ (13).

Figure 1.

A dilemma of the human HO-1 gene in its transcriptional regulation. Note a feedback regulation involving HO-1, Bach1, and heme. The composite enhancer of the human HO-1 gene, containing cadmium-responsive element (CdRE) and a putative Maf recognition element (MARE), are schematically shown. The putative MARE may be bound by heterodimers, consisting of Nrf2 or Bach1 and one of small Maf proteins. Also shown are potential HSE, the polymorphic (GT)n repeat, and E boxes in the proximal promoter region (1, 4, 11).

Footnotes

This study was supported by Grants-in-aid for Scientific Research (B), for Exploratory Research, and for Scientific Research on Priority Areas from the Ministry of Education, Science, Sports and Culture of Japan, and by the grant provided by the Uehara Memorial Foundation.

1

To whom requests for reprints should be addressed at Department of Molecular Biology and Applied Physiology, Tohoku University School of Medicine, 2-1 Seiryo-machi, Aoba-ku, Sendai, Miyagi 980-8575, Japan. E-mail:

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